So-called "smart battery" technology involves a current monitoring unit that monitors the current in a precision resistor connected in series with a battery system and an electrical load to monitor the quantity of electrical charge transferred to or from the battery system. The current monitoring unit measures the charge and discharge current of the battery system and, using an accurate time base, calculates the time integration of charge and discharge current flowing through the battery system. The current time integration can be further enhanced by the use of mathematical models of the charge and discharge efficiencies of the battery system. The unit may also implement a correcting calculation that takes into account the self-discharge rate of the battery system.
The overall accuracy of this approach, which is commonly referred to as "coulomb counting", depends on the accuracy of the current measurement across the fill operating range of the battery system, on both the charging and discharging of the system. Typically, the current measuring device measures the voltage across a shunt resistor connected in series with the battery system and converts the measured voltage to a current, which current is used to determine the state of charge of the battery system. Such a measurement scheme performs adequately in low current systems such as laptop computers, cellular telephones and camcorders where there is little electromagnetic noise in the system which can cause errors in the current measurement. For these small, portable electronic devices the battery power and current are relatively small, in the range of approximately 50W to 100W and 1A to 5A, respectively. The use of smart batteries is widely accepted in these portable systems. The typical smart battery device for these smaller systems employs a small precision shunt resistor, on the order of approximately 10 to 25 m.OMEGA. with a 1W rating, mounted directly on a printed circuit board (PCB). This results in a compact and easily assembled device.
For larger battery systems employed in applications such as automotive starting, lighting and ignition (SLI), electric vehicles, hybrid electric vehicles and stationary power backup battery systems, however, the charge and discharge currents are substantially greater in magnitude than for portable electronic devices. For example, SLI batteries may be required to draw as much as 600A during vehicle starting. The larger current levels require substantially lower shunt resistor values and higher power dissipation ratings. The low resistance of such a shunt results in a very small voltage drop across the shunt that must be measured in order to determine the charge and discharge current in the battery system. In some cases, the voltage drop that must be measured can be as low as 10 .mu.V.
The requirement to dissipate greater amounts of heat prevent the direct mounting of the shunt resistor on a PCB due to both heat dissipation considerations as well as required conductor cross sections to carry these high currents. A current sense resistor is typically mounted remotely from the electronics that measure, convert and/or analyze the shunt resistance voltage drop which is proportional to the current in the shunt resistor. Such an arrangement requires that relatively long conductors or wires be utilized to make the electrical connection between the current sense resistor and the electronic monitoring circuitry. These long conductors or wires cause the low voltage measurements to be subject to induced noise in the presence of a varying electromagnetic field, typically in the form of electromagnetic interference (EMI). The EMI induced in the conductors is proportional to the conductor length as given by the following relationship for the electromotive force (EMF) induced in a stationary conductor exposed to a time varying magnetic field: ##EQU1##
where E is the electric field intensity, dl is the conductor length and B is the magnetic flux normal to the conductor surface.
Since the function of the battery monitoring system is to provide a time integration of the battery current in order to track the battery state of charge, even small errors in the measurement of the shunt current due to the EMI in the conductors can cause large errors in the state of charge measurement to accumulate over time. These errors render the accurate state of charge monitoring of battery systems in the presence of EMI difficult or impossible with existing current sensor approaches